Solar cells have been attracting attention as an environment-friendly electric energy source and as an energy source which is effective against the energy problem with increasing seriousness currently. Now, inorganic substances, such as monocrystalline silicon, polycrystal-line silicon, amorphous silicon and compound semiconductor, are used as a semiconductor material of a photovoltaic device of a solar cell. However, solar cells produced using inorganic semiconductors have not spread widely for home use because of their cost higher than that of power generation systems such as thermal power generation and nuclear power generation. Such a high cost is derived mainly from the process of producing a semiconductor film in vacuum at high temperatures. In such situations, organic solar cells using organic semiconductors such as conjugated polymers and organic crystals or organic dyes have been investigated as semiconductor materials with which the production process is expected to be simplified.
However, the most serious problem with organic solar cells using conjugated polymers and so on is that such solar cells are low in photoelectric conversion efficiency in comparison with solar cells using conventional inorganic semiconductors, and therefore such solar cells have not been used practically, yet. The following two points are major reasons for the low photoelectric conversion efficiency of organic solar cells using conventional conjugated polymers. The first reason is that a bound state called exciton in which an electron and a hole generated by incident light are resistant to separation is formed. The second reason is that since a trap which captures a carrier (electron or hole) is likely to be formed, a formed carrier tends to be captured by the trap, resulting in low mobility of carriers. In other words, while a semiconductor material is generally required that the carriers of the material has a high mobility μ, there is a problem that conjugated polymers have mobilities μ being lower than those of conventional inorganic crystalline semiconductors or amorphous silicon.
Therefore, finding the means for successfully separating a formed electron and a formed hole from exciton and the means for preventing carriers from scattering between amorphous regions of a conjugated polymer or between conjugated polymer chains or from being captured by the trap to increase the mobility is the key for bringing solar cells using organic semiconductors in practical use.
The hitherto known photoelectric conversion devices can now be generally classified into the following elemental constitutions; that is, a Schottky type in which an electron donating organic material (p-type organic semiconductor) and metal with a small work function are joined, and a heterojunction type in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are joined, and so on. In such devices, since only organic layers (almost several molecular layers) in a junction contribute to the generation of a photocurrent, the photoelectric conversion efficiency is low and, therefore, the improvement in the efficiency is a pending problem.
One approach for increasing the photoelectric conversion efficiency is a bulk hetero-junction type in which an electron accepting organic material (n-type organic semiconductor) and an electron donating organic material (p-type organic semiconductor) are mixed so that joined surfaces which contribute to photoelectric conversion are increased (see, for example, J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglla, R. H. Frirnd, S. C. Moratti, A. B. Homes, “Nature,” No. 376, p. 498, 1995). In particular, photoelectric conversion materials using a conjugated polymer as an electron donating organic material (p-type organic semiconductor) and using fullerene such as C60 or carbon nanotubes as well as a conducting polymer having an n-type semiconductor property as an electron accepting organic material have been reported (see, for example, E. Kymakis, G. A. J. Amaratunga, “Applied Physics Letters” (USA), Vol. 80, p. 112, 2002, G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, “Science,” Vol. 270, p. 1789, 1995 and Japanese Unexamined Patent Application Nos. 2003-347565 (Claims 1 and 3) and 2004-165474 (Claims 1 and 3)).
Moreover, a photoelectric conversion material has been reported which comprises a conjugated polymer with a band gap having been reduced by the introduction of an electron donating group and an electron attracting group into a main chain to cause the polymer to efficiently absorb the radiant energy of a wide range of the sunlight spectrum (see, for example, US Unexamined Patent Application Publication No. 2006/174937 specification). Strenuous researches are made to thiophene skeletons as the electron donating group and to benzothiadiazole skeletons as the electronic attracting group (see, for example, X. Li, W. Zeng, Y. Zhang, Q. Hou, W. Yang, Y. Cao, “European Polymer Journal,” Vol. 41, p. 2923, 2005, Japanese Unexamined Patent Application Publication No. 2003-104976 (Claims 1 and 9) and US Unexamined Patent Application Publication Nos. 2004/115473 specification and 2006/52612 specification). However, sufficient photoelectric conversion efficiency has not been obtained, yet.
As described above, such conventional organic solar cells are problematic low in photoelectric conversion efficiency. It could therefore be helpful to provide a photovoltaic device with high photoelectric conversion efficiency.